Please don't read Gibbon to learn anything about Rome unless it's what 18th century British scholars think of it. He is important for the historiography of the field but wrong about a lot of things, especially anything about religion and the Byzantine Empire.
It would be shocking if we hadn't learned more about the subject, since, but the vast majority of source material we now have was available to Gibbon and well-digested by him. It's perfectly evident reading Gibbon that he couldn't be straightforward about religion at that time.
"a degree of professional esteem which remains as strong today as it was then", from:
"In the early 20th century, biographer Sir Leslie Stephen ["Gibbon, Edward (1737–1794)," Dictionary of National Biography, vol. 7, (Oxford, 1921), p. 1134.] summarized The History's reputation as a work of unmatched erudition, a degree of professional esteem which remains as strong today as it was then:
The criticisms upon his book...are nearly unanimous. In accuracy, thoroughness, lucidity, and comprehensive grasp of a vast subject, the History is unsurpassable. It is the one English history which may be regarded as definitive. ...Whatever its shortcomings, the book is artistically imposing as well as historically unimpeachable as a vast panorama of a great period."
